So, I can't believe I never thought to wonder about this before, but.....what exactly is the difference between a lunar eclipse and a new moon? Both are basically the moon being in the earth's shadow, and thus not being able to reflect the light of the sun, correct?

Dear xkcd,

On behalf of my religion, I'm sorry so many of us do dumb shit. Please forgive us.

no, a new moon happens when the moon is lit from behind. Just like a full moon happens when it's lit from front, and a half moon when the light comes from 90°.Actually, the solar eclipse is a new moon, in which the moons shadow falls on earth. During ordinary new moons, the moons shadow simply does not hit earth.

The moon orbits earth at an angle. When the moon is between the sun and the earth it's usually not directly between the sun and the earth (eclipse). Instead it's at an angle, lit from behind by the sun but not blocking it.

Sun ----//-------------------------- Earth -- MoonMoon is Full (lit side almost entirely towards the Earth, hard to differentiate the lit border from the actual visible horizon of the Moon) but has a chance of passing exactly1 into the shadow of the Earth, blocking out the sunlight and becoming a Lunar Eclipse.

Sun ----//--------------Moon -- EarthMoon is New (lit side almost entirely away from the Earth, hard to spot the lit border lapping over the visible horizon of the Moon) but has a chance of passing exactly2 between the Earth and the Sun so as to cast its shadow onto the Earth and becoming a Solar Eclipse.

As already said, because the Moon orbits on a plane slightly different from the one in which the Earth orbits the Sun, the position of the Moon as it crosses across the Sun's track does not match directly up with the position of the Sun as it crosses across the Moon's track every time (for both Sun-Moon-Earth and Sun-Earth-Moon sides of the equation). And though Lunar Eclipses must happen at times of Full Moons and Solar Eclipses must happen at times of New Moon, not every Full/New Moon (maximally lit/unlit point, for this orbit) has a Lunar/Solar eclipse.

1Fairly exactly. There's leeway, because the Earth is much bigger than the Moon and the umbra is wider than the Moon at the distance it orbits. But the entire night-side of Earth has opportunity to see the Moon be obscured and unlit, save by fThe light of a whole-world's worth of sunsets/sunrises/horizon-scrapes.2 Much more exactly. The Moon is smaller than the Earth and the umbra gets down to a very small point that only gives (at best) a very small track across the world the Totality experience. If the Moon is slightly further out in its orbit (or you're too far around the Earth, even though directly on the track extending away from the ends) the shadow's point is above you, and you just get to see a slightly-too-small Moon creating an Annular Eclipse. If it is not an eclipse and you're seeing the Moon from just over the 'solar horizon' (day/night boundary), you may see the 'dark' side of the (low in the sky) Moon lit at a very low level by the Earthshine of the whole Sun-facing side of the planet.

Aikanaro wrote:So, I can't believe I never thought to wonder about this before, but.....what exactly is the difference between a lunar eclipse and a new moon? Both are basically the moon being in the earth's shadow, and thus not being able to reflect the light of the sun, correct?

I like the diagram (you can match it with my text), but I don't get why we're apparently looking over the North Pole of Earth, yet the ecliptic plane (and the angle off of it) assumes that we're viewing from the side somewhere within the tropics... Looks like it was repurposed from a top-down view.

Still, it does more than my ASCII.

(And I know when I was young, I always got mixed up between whether Lunar Eclipse was an eclipse of the Moon or by the Moon. It's the former. I justified it to myself by remembering that the other type was Solar (of the Sun, by the Moon) not Earthly/whatever (by the Earth, of the Moon), given that we're not normally in a position to view anything whilst standing upon either Sun or Moon to confuse everything. Well, that was my ~8yo self's mnemonic. Maybe it works for someone else, maybe not. But I give it you for free, anyway...)

That diagram is confusing me: the ecliptic plane is depicted as a line and the angle is depicted as (kind of) constant, implying a side-view of the Solar system (as it should be), but the Earth is depicted with the North Pole at the centre, implying a top-down view of the Solar system (which it shouldn't be).

Actually, the diagram isn't confusing me, but I'm curious how that error slipped into the diagram.

If the moon orbits at an angle, why does it change angle to get directly in the sunlight's path? If it's changing angle, why doesn't it eventually progress to orbiting perpendicular to the ecliptic, and then back again?

Jatopian wrote:If the moon orbits at an angle, why does it change angle to get directly in the sunlight's path? If it's changing angle, why doesn't it eventually progress to orbiting perpendicular to the ecliptic, and then back again?

As I understand it, the moon always orbits at (more or less) the same angle, but it's inclined orbit doesn't turn as the earth orbits the sun, so twice a year the places where it crosses the Earth's orbit also line up with the sun. At these times eclipses are possible if the moon is in the correct phase.

(Much as said by measure, but I'm gonna use different words…) The Moon's tilt is like a gyroscope in its ability to hold an angle1. But it's a gyroscope standing on the rim of a roundabout/carousel, the Sun at the centre. If you sit with the gyroscope and concentrate on the Sun, you are rotating, but if you see the Sun as steady then you'd see the gyroscope's tilt as rotating and (effectively) flapping about. But always the same degree of tilt, just a changing direction in which that angle is. It'll never go perpendicular (if it isn't perpendicular in any manner to start with).

1 Gyroscopes do also precess (wobble), and so do all the various orbital mechanics like the Earth spinning and (in this example) the Moon's plane of orbit, but compared to the spin rate that is a very slow readjustment that doesn't matter in this analogy.